As the Internet has become widely used and information services are provided through networks, the communication network occupies an important place in social infrastructure. With the increase in Internet access from homes and business locations of companies, faster and higher-capacity access lines are increasingly required that interconnect these communication sites and the communication stations of carrier networks.
As one of the access networks to be connected to a wide area network such as the Internet, there is a passive optical network (PON) in which a plurality of subscriber terminals can share an optical fiber. The PON systems are each installed at a user's site, and each system includes a plurality of optical network units (ONU) containing one or plural user terminals, and an optical line Terminal (OLT) connected to these ONUs via optical fiber networks. Optical fibers connected to the OLT are joined with branch optical fibers connected to each ONU by means of an optical splitter (optical coupler), and the optical transmission channel between the optical splitter and the OLT is shared by a plurality of ONUs (user terminals), thereby reducing the optical fiber installation cost substantially.
Known PON systems include the B-PON (Broadband PON) wherein information is transmitted by fixed-length ATM cells over the optical fiber section (PON section), the G-PON (Gigabit PON) enabling a fast data transfer on the order of gigabits, and the GE-PON (Gigabit Ethernet PON) suitable for an information transmission by Ethernet frames that is becoming popular for LANs and metro-networks.
The G-PON and the GE-PON allow variable-length frames to be transferred in the PON section, and their standardization and technological assessment is being made by the ITU-T and the IEEE respectively. ITU-T recommendations on the G-PON include the non-patent documents 1 through 3 below, wherein the GEM (G-PON Encapsulation Method) frame specification is defined as a transmission frame specification for transferring common variable-length frames, not restricted to Ethernet frames, in the PON section.
In a PON system, downstream frames from OLT to ONU are branched into plural branch optical fibers by a splitter to be delivered to all ONUs. Each ONU determines whether or not a received PON frame is one to be processed by itself, based on the destination identification information in the header (such as a GEM header), of the received PON frame. Upstream frames from ONU to OLT are multiplexed into the OLT-side optical fiber by an optical splitter. In upstream communications, in order to prevent overlapping of frames on the optical fiber, the TDMA scheme is employed that causes each ONU to send frames during a transmission time interval allocated by the OLT.
As can be understood from the above configuration, the PON system can be an access network suitable for delivering the same service information to a plurality of user terminals by multicasting it from the OLT. Accordingly, for the triple-play service (broadcasting, telephone, and data communications), which is drawing attentions recently, particularly for the broadcasting industry seeking to enter into the network business, the PON system plays an important role as an access network.    Non-patent document 1: ITU-T G.984.1 “Gigabit-capable Passive Optical Networks (GPON): General characteristics”    Non-patent document 2: ITU-T G.984.2 “Gigabit-capable Passive Optical Networks (GPON): Physical Media Dependent (PMD) layer specification”    Non-patent document 3: ITU-T G.984.3 “Gigabit-capable Passive Optical Networks (GPON): Transmission convergence layer specification”